The present invention relates to methods and apparatus for sensing touch events on a touch sensitive display, such as a liquid crystal display, organic light emitting diode display, etc.
The display market is prime for displays that offer touch sensing capability—and the market size for displays with touch functionality is expected to grow tremendously in the coming years. As a result, many companies have researched a variety of sensing techniques, including resistive, projected capacitive, infrared, etc. While many of these techniques result in reasonable touch capability, each technique carries some performance disadvantage for specific applications, and nearly all result in significant added cost to the manufacture of each display.
In terms of performance, the basic metrics for touch sensitive displays are the accurate sensing of a touch event and the determination of the precise location of the touch event on the touch/display window. Many secondary attributes are becoming important for added functionality, including flexibility in sensing various touching implements beyond the human finger, such as a pen, stylus, etc., the ability to sense multiple, simultaneous touch events, location resolution, and the ability to distinguish false touches (hovering, or environmental disturbances).
As touch sensitive displays are gaining wider use in mobile device applications, such as the iPhone™, iPOD™, etc., the overall thickness and weight of the touch sensitive display are becoming more important metrics for commercial viability. When such additional criteria are taken into consideration, very few sensor technologies stand out.
Currently, resistive touch-screens dominate the market because of their scalability and relatively low cost. A common variety of resistive touch-screen is the 4-wire type, where two un-patterned transparent conductors (typically coatings of Indium tin oxide, ITO) face one another, one on the underside of a plastic film and the other atop a glass substrate. Voltage is alternately applied to each conductor on opposing edges. Due to resistance within the conductors, a voltage drops across the sheet. When voltage is not applied to a given sheet, that sheet acts as a sensor. When the plastic film is displaced via a touch event, the two conductive sheets come in contact with one another and current flows from the energized sheet to the non-energized sheet. The voltage at the point of contact depends upon the distance from the input source, which allows the position of the contact to be determined in one dimension. By reversing the source and sense roles of the two sheets, the position can be similarly determined in the other dimension.
There are number of disadvantages associated with resistive touch-screens, such as the plastic film being relatively prone to damage, the ITO coating being prone to cracking (as such coating is fairly brittle), and the ITO coating neither being as transparent nor conductive as desirable. Resistive touch-screens are also unable to support multi-touch capability first popularized in the iPhone™ (which uses a capacitive touch-screen). The very light touch possible on the iPhone is also not possible on a resistive touch-screen because the film must be physically displaced to bring the two ITO coatings into contact.
Although more costly than the resistive variety, capacitive touch-screens are becoming more popular. A capacitive touch-screen includes a touch glass layer and a cover glass layer. The touch glass layer carries electrical traces (usually ITO) on opposing sides, usually in a crossing grid pattern with an insulator (the glass) therebetween. As the human body is a conductor, touching the cover glass results in a distortion of the local electrostatic field, measurable as a change in capacitance. A square wave is sequentially input into each electrical trace in a given direction, and the mutual capacitive coupling to each of the lines in the other direction is sensed. If a finger touches the cover glass, the mutual capacitance will be reduced at more than one unit cell (the crossing of respective traces of the grid). Capacitive touch-screens are desirable due to their ability to provide multi-touch sensitivity, their ability to sense even very light touch events, their robustness (no flexing is required), and their transparency characteristics.
The drawbacks of capacitive touch-screen technology include: the inability to sense the touch of a stylus, the difficulty in scaling to larger sizes, and the high cost of manufacture.
Accordingly, there are needs in the art for new methods and apparatus for advancing touch-screen technology to include: good scalability, low cost, sensitivity to a stylus, robustness, good transparency, sensitivity to multi-touch events, and sensitivity to light touch events.